1. Field of the Invention
The present invention relates to a lithium polymer battery using a polymeric electrolyte, and more particularly, to a lithium polymer battery having improved charging/discharging efficiency and life cycle characteristics.
2. Description of the Related Art
Secondary batteries are essential components of various portable electronic devices and telecommunications equipment, such as portable audio devices, cellular phones, camcorders, notebook type computers and the like. Thus, there is an increasing demand for small, lightweight secondary batteries capable of charging and discharging so as to supply power to such devices and equipment.
Secondary batteries that are prevalently being developed and used include Ni—Cd batteries, Ni-MH batteries or lithium batteries. Lithium batteries are characterized by their excellent characteristics, such as long life span, high capacity and so on, and are highly promising as a next generation power source.
A liquid electrolyte is generally used in most conventional lithium secondary batteries. However, the use of the liquid electrolyte frequently causes damage to devices due to a leakage of the electrolyte solution. The leakage allows the inside of the battery to dry due to a volatile solvent and results in a shorting between electrodes.
To overcome the foregoing disadvantages, a solid electrolyte has been proposed in place of the liquid electrolyte. Solid electrolytes are generally free from leakage of the electrolytic solution and are easily processible. Thus, research into the solid electrolytes, in particular, polymeric electrolytes, is actively being carried out.
The ionic conductivity of a polymeric electrolyte has a great effect on the internal resistance of a lithium battery during charging and discharging, and further contribute to the efficiency of the battery. Thus, there is a need for an electrolyte that is capable of preventing shorts within the battery, while maintaining a high ionic conductivity by impregnating a large amount of an electrolytic solution and allowing a high mobility of the lithium ions.
Polymeric electrolytes are classified into pure polymeric electrolytes having a lithium salt retained into polymer, and polymeric gel electrolytes having an organic electrolytic solution and the polymer. The polymeric gel electrolytes are developed in an attempt to attain a gelled polymer so as to make the organic electrolytic solution non-flowable. Around 1990, polyethylene oxide, polyacrylonitrile and polysiloxane have been introduced as host polymers in U.S. Pat. No. 4,830,939 and in Japanese Patent Laid-open Publication No. Hei 5-109310. However, the polymeric gel electrolyte prepared from these polymers was still inadequate to be used in practice in terms of their ionic conductivity (i.e., approximately 10−4 S/cm).
In the middle of 1990, methods of impregnating an electrolytic solution in a large amount by decreasing the crystallinity of polymers by copolymerization (i.e., by using a vinylidene fluoride (VdF) and hexafluoropropylene (HFP) copolymer or an acryloyl allyl copolymer) by which the ionic conductivity of the electrolytic solution has improved to a practical level of 10−3 S/cm.
Hitherto, gel electrolytes having the same composition have, however, been applied between a cathode and an anode. Thus, the improvement of charging/discharging efficiency and life cycle characteristics is still not satisfactory. In other words, since the charging/discharging characteristics of the secondary battery are attributed to electrical oxidation-reduction reactions taking place at the cathode and anode, reaction conditions cannot be appropriately adjusted due to the gel electrolyte having the same composition applied to the cathode and anode. This occurs despite pH levels adjusted to optimize the reactions. Thus, the deterioration in the charging/discharging efficiency and the life cycle characteristics frequently occurs.